Not Applicable
1. Field of Invention
This invention relates to the field of systems for position measurements. Specifically, this invention relates to systems for monitoring the position of a survey unit with respect to a base station using local acoustic time-of-flight measurements.
2. Description of the Related Art
It is known in the art to use magnetic and radiometric environmental surveys to search property for unexploded ordinance and radiation contamination. Such surveys are improved by collecting high resolution, position-correlated survey data which fully document the survey results and permit computer modeling and analysis to identify minimal targets and characteristics. A basic environmental survey is conducted by surveying the area using an appropriate survey instrument and placing a marker where the audible output from the instrument directs. Documentation of such a survey typically consists only of field notes and hand drawings. Such a survey leaves considerable uncertainty as to the reliability of the survey results.
One method for improving the accuracy of position data obtained from a walk-over environmental survey is differential global position system (DGPS) tracking using orbital satellites. DGPS is a convenient method for conducting a survey in open areas; however, it is inaccurate in wooded areas or areas containing structures due to the attenuation and the reflection, or multipathing, of the satellite signals. Further, obtaining the high resolution and accuracy necessary to locate small targets using DGPS is expensive.
Other technologies have been developed to improve the accuracy of walk-over environmental surveys. One such improvement automatically tracks the surveyor and simultaneously records the sensor output in a digital memory. The data is used to provide track maps which document precisely the manner in which the surveyor covers the area. This data is also used to generate color-coded maps showing, in detail, the sensor output over the surveyed area.
Finally, laser tracking has been evaluated, but it is difficult to use in wooded environments where the line of sight is obstructed. When the surveyor""s signal is lost due to tree shadowing, the tracker must reacquire the surveyor""s location before the survey data can be matched. Reacquisition times of several seconds are not uncommon in such environments, thereby greatly slowing the survey and/or reducing the accuracy.
Other devices for ranging determination have been developed. Typical of the art are those devices disclosed in U.S. Pat. No. 4,924,450 issued to Brashear et al., on May 8, 1990. Brashear et al., teaches the use of setting up multiple ultrasonic receivers at fixed locations around the property being surveyed and carrying a backpack containing an ultrasonic transmitter and a radio frequency transmitter. The ultrasonic signal is used to calculate time of flight data from the position of the backpack to each of the ultrasonic receivers. The radio frequency signal carries data from the survey instrument to a computer which is located nearby. Upon receipt of the ultrasonic signal, each ultrasonic receiver announces the arrival time to the computer using a radio frequency signal. The arrival time data is used to calculate the location of the backpack using time-of-flight triangulation.
The invention of Brashear et al., requires that the ultrasonic receivers be located in fixed positions around the property to be surveyed. These positions must be taught to the computer so that the computer can triangulate a position using a value received from a particular ultrasonic sensor. In order for the invention of Brashear et al., to operate properly the distances between multiple ultrasonic receivers must be carefully measured. This makes it unnecessarily difficult to transport and setup the positioning system for use in multiple locations due to the use of multiple receiver units and the precise placement required for proper operation. Finally, the invention of Brashear et al., does not consider the environmental effects of temperature and wind speed on the instantaneous speed of sound for a given measurement. In addition, the commercial version of the Brashear et al. invention is expensive, with the price of the complete system approaching $100,000. Further, it is a complex system requiring a team of experienced operators for use in field operations.
Similarly, U.S. Pat. No. 5,280,547 issued to Figueroa et al., on Jan. 18, 1994 discloses a position detecting system and method. Figueroa et al., teaches the use of a plurality of ultrasonic receivers located at known, fixed locations. In order to eliminate the speed of sound as a variable from distance measurements, a complex mathematical matrix is used to estimate the speed of sound for every ranging operation. Like the system of Brashear et al., the invention of Figueroa et al., requires the calibration of the system to determine the exact location of the receivers. This is accomplished by moving the receiver to a known position and measuring the time-of-flight to each receiver. However, Figueroa et al., does not teach a method for removing the effects of wind from the calculations.
Accordingly, there is a need for system which can be used for environmental surveys and which automatically tracks the surveyor using a local positioning system employing ultrasonic time-of-flight technology and which does not disturb the surveyor or the animals in the area. The system would have high resolution and accuracy, be inexpensive, be lightweight and easily portable, and be suitable for use in most terrains including wooded areas and inside buildings. Further, there is a need for a system which does not require a complex installation and calibration procedure. Such a system would have a single reference station which must be placed to provide a reference point from which to calculate the location of the roving transmitter. Finally, the system would automatically record the current speed of sound and wind effects at each location.
Therefore, it is an object of the present invention to provide a local-positioning environmental survey system employing ultrasonic time-of-flight technology.
Another object of the present invention is to provide a local-positioning environmental survey system in which the ultrasonic pulse does not disturb the surveyor or the animals in the area.
It is another object of the present invention to provide a local-positioning environmental survey system having high resolution and accuracy.
It is a still further object of the present invention to provide a local-positioning environmental survey system suitable for use in most terrains including wooded areas and inside buildings.
Yet another object of the present invention to provide a local-positioning environmental survey system which is cost effective.
Yet still another object of the present invention is to provide a local-positioning environmental survey system which is lightweight and easily portable.
A local positioning system (LPS) using acoustic time-of-flight and a fixed array of receivers provides an automatic means of determining the position of a roving transmitter, or rover, with respect to that of a fixed reference station. The LPS uses local, pulsed, ultrasonic emissions without the need for a clear line of sight between the rover and the reference station. To provide a more accurate calculation of the location of the rover, the position data is adjusted for the current speed of sound and the wind velocity.
The reference station is a stationary device having a plurality of equidistant receiver pods mounted on the distal end of elongated members radiating outwardly from a control module in an orthogonal array. The receiver pods are designed to receive information transmitted from the roving transmitter. A leveling system is included to level the horizontal receiver pods. To ensure clear reception of the ultrasonic transmissions, it is desired that the rover be aimed at the reference station. The optional transmitter pod configured to broadcast the ultrasonic signal in a circular pattern can be connected to the rover eliminating the need to aim the rover.
The roving transmitter is designed to be small, lightweight, and easily portable, either worn or carried by the user. The rover emits a short ultrasonic pulse, lasting only a few cycles, in a near circular wavefront pattern. The time of the emission of the ultrasonic pulse is announced to the reference station using a low power radio frequency signal.
Various connectors are included to allow the rover to communicate with external devices. For example, some survey instruments contain sufficient memory to allow the LPS data to be stored along with the data acquired by the survey instrument. In other cases, it is preferable to channel the data acquired by the survey instrument through the rover and output the survey instrument data, along with the LPS data, to a portable memory device. Once transferred, the LPS data can be translated into coordinates using a simple post-processing program.
To calculate the position of the rover in relation to the reference station, a short ultrasonic pulse is emitted from the rover ultrasonic transmitter. Simultaneously, the rover RF transceiver transmits a low power RF signal pulse. When the RF signal pulse is received by the reference station RF transceiver, the reference station starts a clock for each receiver pod. As the leading edge of the ultrasonic pulse is received at each receiver pod, the associated clock is stopped. The processing device records the measured time-of-flight from each of the receiver pods.
To measure the speed of sound for the current atmospheric conditions, an ultrasonic pulse is emitted from a transmitter centrally mounted on the reference station, equidistant from each receiver pod. The speed of sound is calculated by averaging the time-of-flight of this ultrasonic pulse to each pair of diametrically opposed receiver pods and dividing into the distance from the reference station ultrasonic transmitter to the receiver pods. By averaging time of flight value to two diametrically opposed receiver pods, the effect of wind along that axis can be removed. The reference station transmits the measured times to the rover via the RF transmitter.
The accuracy of the measurements is improved by incorporating an orthogonal array of four receiver pods thereby making it always possible to choose three pairs of receiver pods where the angle between the location of the rover and the normal bisector to the line joining each receiver pod pair is less than 45 degrees. To further reduce the sensitivity of the LPS to time-of-flight measurement errors and improve the accuracy of the LPS, the data obtained from the three preferred receiver pod pairs are averaged.